Indoor Thermostat

A building thermostat is a device to control the heating and air-conditioning systems in a home, office, or other indoor environment.

The majority of modern heating/cooling/heat pump thermostats operate on 24 volts A/C.

 

Common HVAC Thermostat Wiring

(R) Red - Voltage

Transformer provided DC voltage

(G) Green - Blower Fan

Many thermostats (G) and (Y) terminals are connected together at all times when the fan switch is in the "Auto" mode.

(Y) Yellow - Compressor

The Air Conditioner. When voltage supplied it activates cooling. On a Heat Pump system it can also activate heat via the heat pump.

(W) White - Heat

Activate heat. Heat including gas heat, electric heat, or auxiliary heat on a heat pump.

(B) Blue - Common (exceptions exist)

What is called 'common' and is typically not used on modern thermostats.

Needed on some electronic thermostats or if the system uses indicator lamps.

On some Rheem & Ruud systems it is not 'common'. May be Blue or Orange.

 

Hot Wire Testing

Shorting or Hot Jumping is risky and can in face damage your system. Certain brand and models work differently from what is commonly known. Use this as a general reference but do not proceed unless you have research your system brand and model.

5 Wire Red, Blue, White, Green, Yellow

• Red + Yellow - turns on the Air Conditioner
• Red + Green - turns on the blower fan
• Red + White - turns on heat

 

4 Wire 24V HVAC Thermostat Wire Diagram

W : White Wire : Heat

Y : Yellow or Black Wire : Cool

G : Green : Fan

R : Red : 24v

Jump W + R will turn on heater on a gas operated HVAC system. On the gas system the fan is triggered automatically at the heater when a certain temperature is reached.

Jump G + R to force the fan to turn on

Honeywell 6 Connector

Honeywell's round mechanical mercury thermostat "The Round" has 6 connectors. Only 4 are necessary for a typical HVAC system.

B : black

G : green : Fan

Y : yellow : Cool

W : white : Heat

R : red : 24v

O : orange

Modern 8 Connector Thermostat

A modern electronic thermostat may have as many as 8 connectors. To connect this to a 4 wire 24v system use the following:

W : white : heat

Y : yellow or black wire : cool

G : green : fan

B :

O :

RC : red wire : 24v - bridge to RH

RH : red wire : 24v - bridge to RC

C :

 

2 Wire and 3 Wire Systems

On a 2 wire installation: If using a mercury thermostat make sure it is level. The red wire from the heater or air conditioner control is mounted to the "R" terminal on the backing plate. The white wire from the heater or air conditioner control is mounted to the "W" terminal on the thermostat mounting plate.

On a 3 wire (series 20) installation: The Red wire coming to the thermostat from the heater or air conditioner connected to "R". The white wire connected to the "Y" terminal. The blue wire connected to the "W" terminal.

Heatpump

The reversing valve wire for cooling is typically orange (but is occasionally blue) and labelled O or R. This connects to the O terminal. Your thermostat may also have the reversing valve wire for heating, labelled B, which should be connected to the B terminal. If you don't have this wire, cap the terminal with a wire nut. The fan control relay wire is typically green and labelled F or G, and should be connected to the G terminal.

The second stage models cooling circuit wires are typically blue and labelled Y2. The hearing wires are usually pink and labelled W1, W2, or W-U. These must be connected to their terminals.

COMMON HEATPUMP BRANDS

These systems are more common and power the reversing valve in A/C mode. (O Terminal)

Orange is the OG reverse valve

To Jumpstart the A/C connect RED the voltage to GREEN, YELLOW, and ORANGE

• Green for the fan, Yellow for the Compressor, and Orange for the OC reversing valve.

To Jumpstart the Heat connect RED the voltage to GREEN and YELLOW

• The heat pump is operating for heat mode

IF IT IS A RUUD OR RHEEM SYSTEM

These systems power the reversing valve in heat mode (E Terminal)

To Jumpstart the A/C connect RED the voltage to GREEN, and YELLOW

• Green for the fan, Yellow for the Compressor, and Orange for the OC reversing valve.

To Jumpstart the Heat connect RED the voltage to GREEN, YELLOW and ORANGE

• The heat pump is operating for heat mode

Wiring Example for Thermostat for a single stage heat pump with aux backup emergency heat

• one stage heat
• one stage cool
• backup emergency heat

C = common = black wire
Rc and R = cooling power, heating power = red wire
O/B = Change over valve aka reverse valve = orange wire
Y = compressor stage 1 = yellow wire
G = fan relay = green wire
AUX-E = Backup Heat/ Emergency Heat = white wire

Additional:
S1 = outdoor sensor = gray 
S1 = outdoor sensor = pink